American Fastener Journal July-Aug : Page 48

WHY SPECIFY ISO STANDARDS FOR METRIC FASTENERS? by Joe Greenslade Metric fastener standards have been confusing in the United States since the mid-1970s. It is time for the confusion to stop. The American Society of Mechanical Engineers (ASME) and the American Society of Materials and Testing (ASTM) are in the process of withdrawing their metric standards, as did DIN in 2001, in preference for ISO standards. One international metric fastener system is all that is needed and is the only logical way to proceed. The time for the universal adoption of ISO standards by end users and suppliers in the United States has arrived. GENERAL MOTORS, FORD, AND CHRYSLER SPEARHEADED THE ADOPTION OF THE METRIC SYSTEM IN THE U.S. IN THE EARLY 1970S. The United States started moving into the use of metric fasteners in a significant way in the early 1970s when the “Big Three” (Ford, GM, and Chrysler) made a commit-ment to use the metric system for all new vehicle designs. Because the automotive industry still uses more threaded fasteners than any other industry, the major fastener suppliers in the USA started getting involved in the production of metric fasteners. This auto industry commitment to metric design was adopted so that one car design could be produced all over the world, instead of hav-ing one design for North America and another design for all markets outside North America. The huge international equipment manufacturers, like Caterpillar and John Deere, soon followed suit. Until that time, relatively low volumes of metric fasteners were used in the United States. They were mostly used in mainte-nance applications for the maintenance of imported manufacturing equipment from Europe, with the majority from Germany. The German standards system is referred to as the DIN (Deutsches Institut fur Normung e. V.) system, and most of the imported fas-teners used to support the import equip-ment were made in Europe to the DIN fastener standards. The decision by General Motors, FORD, and Chrysler to adopt the metric system of measurement in design impacted all industri-alized countries in the world. The car manu-facturers wanted to be able to source products anywhere in the world and have the components be compatible regardless of where the parts were made, purchased or assembled. This decision drove the need for the creation and use of common world stan-dards for all components, including fasteners. U.S. ASME/ASTM Metric Standards DIN Metric Standards ISO Metric Standards THE ISO FASTENER COMMITTEE WAS FORMED TO COMMONIZE METRIC FASTENER SYSTEMS. At that time, several European and Asian countries had their own designs of metric fasteners as defined by their country stan-dards. To standardize these designs, the ISO TC2 Fastener Committee was formed. The predominant metric fastener standard at the time was DIN, so it became the foundational metric fastener standard system from which the eventual ISO fastener standards evolved. The United States has sent delegates to the ISO TC2 meetings, but instead of adopt-ing the ISO fastener standards, it was decid-ed the U.S. would create its own set of metric fastener standards through the Amer-ican Society of Mechanical Engineers (ASME), the Society of Materials and Testing (ASTM), and SAE International. The U.S. delegates to ISO TC2 felt they could create a superior metric fastener sys-tem, called the Optimum Metric Fastener System (OMFS). The main goals of OMFS were: 1 Eliminating the recognition of all thread series, except course threads, to mini-mize the number of possible sizes to manage; 2 The addition of a new size, M6.3 X 1.0, to be sure American designers had a size closer to ?-20 than M6 X1, yet not directly interchangeable with ?-20; 3 The reduction of the across flats sizes on hex cap screws and hex nuts by 1 mm on sizes M10, M12, and M14, to save the material content in these part sizes; continued on page 50

Why Specify ISO Standards For Metric Fasteners?

Joe Greenslade

Metric fastener standards have been confusing in the United States since the mid-1970s. It is time for the confusion to stop.The American Society of Mechanical Engineers (ASME) and the American Society of Materials and Testing (ASTM) are in the process of withdrawing their metric standards, as did DIN in 2001, in preference for ISO standards. One international metric fastener system is all that is needed and is the only logical way to proceed. The time for the universal adoption of ISO standards by end users and suppliers in the United States has arrived.

GENERAL MOTORS, FORD, AND CHRYSLER SPEARHEADED THE ADOPTION OF THE METRIC SYSTEM IN THE U.S. IN THE EARLY 1970S.

The United States started moving into the use of metric fasteners in a significant way in the early 1970s when the “Big Three” (Ford, GM, and Chrysler) made a commitment to use the metric system for all new vehicle designs. Because the automotive industry still uses more threaded fasteners than any other industry, the major fastener suppliers in the USA started getting involved in the production of metric fasteners. This auto industry commitment to metric design was adopted so that one car design could be produced all over the world, instead of having one design for North America and another design for all markets outside North America. The huge international equipment manufacturers, like Caterpillar and John Deere, soon followed suit.

Until that time, relatively low volumes of metric fasteners were used in the United States. They were mostly used in maintenance applications for the maintenance of imported manufacturing equipment from Europe, with the majority from Germany.The German standards system is referred to as the DIN (Deutsches Institut fur Normung e. V.) system, and most of the imported fasteners used to support the import equipment were made in Europe to the DIN fastener standards.

The decision by General Motors, FORD, and Chrysler to adopt the metric system of measurement in design impacted all industrialized countries in the world. The car manufacturers wanted to be able to source products anywhere in the world and have the components be compatible regardless of where the parts were made, purchased or assembled. This decision drove the need for the creation and use of common world standards for all components, including fasteners.

THE ISO FASTENER COMMITTEE WAS FORMED TO COMMONIZE METRIC FASTENER SYSTEMS.

At that time, several European and Asian countries had their own designs of metric fasteners as defined by their country standards.To standardize these designs, the ISO TC2 Fastener Committee was formed. The predominant metric fastener standard at the time was DIN, so it became the foundational metric fastener standard system from which the eventual ISO fastener standards evolved.

The United States has sent delegates to the ISO TC2 meetings, but instead of adopting the ISO fastener standards, it was decided the U.S. would create its own set of metric fastener standards through the American Society of Mechanical Engineers (ASME), the Society of Materials and Testing (ASTM), and SAE International.

The U.S. delegates to ISO TC2 felt they could create a superior metric fastener system, called the Optimum Metric Fastener System (OMFS). The main goals of OMFS were:

1 Eliminating the recognition of all thread series, except course threads, to minimize the number of possible sizes to manage;

2 The addition of a new size, M6.3 X 1.0, to be sure American designers had a size closer to ?-20 than M6 X1, yet not directly interchangeable with ?-20;

3 The reduction of the across flats sizes on hex cap screws and hex nuts by 1 mm on sizes M10, M12, and M14, to save the material content in these part sizes;

4 The introduction of a new fixed limit thread gaging system to achieve higher fastener thread quality; and

Since the 1970s, each of these goals has proven to be confusing, impractical and not beneficial. All of these, except the across flats changes, were either never adopted by users, or adopted for a relatively short time and later abandoned.

1 ISO metric fastener threads come in coarse, fine, and extra fine series, just as the U.S. inch system threads always have. The usage of coarse to fine threads of ISO threads is about the same as in the inch threads. The overall usage is in the vicinity of 90 percent coarse, 9 percent fine, and 1 percent extra fine. OMFS had no impact on usage in the long run.

2 The size M6.3 X 1.0 was adopted by General Motors for a relatively short period of time and dropped because it was found to be confusing and had no significant benefits over the original M6 X 1.0. ISO STANDARDS continued from page 48

3 ISO TC2 did adopt the smaller across flats hex size for M10, M12, and M14.These have been employed around the world in some organizations, but many earlier designs were based on the larger across flat that are in the German DIN standard. As a result, many users still specify DIN bolts and nuts. It is the author’s opinion that had ISO stayed with the larger hex sizes, the DIN standard would have been largely eliminated today. Some material was saved, but some confusion in metric fasteners was introduced and persists today.

4 The OMFS thread gaging systems was tried for a few years. The experience of those who tried it could not accept a lot of products with the new systems that were accepted by the old and no improvement in product performance was ever proven by the use of the new system. As a result the OMFS thread gaging systems was quickly abandoned.

5 The 12-spline flange drive design was covered in ASME standards, but it is believed that products using this design were never produced because they were difficult to manufacture, and higher potential torque delivery was never proven to be a significant benefit over the use of the external hex drive.

In the meantime, more and more countries joined the ISO fastener standards efforts, and a true worldwide fastener standards system was created. The growing adoption of ISO fastener standards has resulted in most industrialized countries withdrawing their country-specific standards and formally adopting the ISO standards as their metric fastener system.

The biggest endorsement of the ISO fastener standards was Germany’s official withdrawal of their DIN fastener standards in 2001, which is documented in DIN 918, Supplement 3. Table 1 shows the withdrawn DIN standards and the ISO standards that should now be used instead.

International trade is increasing every day. USA exports are critical for controlling the USA balance of trade and reducing unemployment. USA firms should be adopting ISO fastener standards to ensure what is produced in the USA is acceptable to purchasers outside the USA.

The good news is that with very few exceptions, fasteners made to the ISO standards, the withdrawn DIN standards, and the USA metric standards are interchangeable.The most significant differences are in the AF (across the flats) sizes on M10, M12, M14 and M22 bolts and nuts. The DIN standards specify a one-millimeter larger hex size for M10, M12 and M14 than do the ISO and ASME standards, and the M22 is one millimeter smaller. Fortunately, all designs have the same strength capabilities. The only practical difference is that installers must use different standard driver socket sizes, depending on which metric standards are used in manufacturing the bolts or nuts.

A concerted effort is being made in the ASME B18 and ASTM F16 fastener committees to start systematically withdrawing their metric fastener standards. Users will be directed to the comparable ISO metric standard.Table 2 shows the ASME metric fastener standards that have been withdrawn so far.

In late 2011, ASTM F568M was withdrawn to encourage the adoption of ISO 898-1. In future action, the ASTM F16 Committee will undertake the withdrawal of ASTM F738M and encourage the use of ISO 3506-1.

U. S. metric fastener suppliers and end users should transition into the use of ISO standards.

It is understood today that a lot of the stocked imported metric fasteners are made to the DIN standards. The good news is that many of the product requirements are identical in the DIN and the ISO standards. None of the differences are known to adversely affect fit or function. Importers should start working with their suppliers to transition from following the DIN standards to following the ISO requirements. For many years, Bossard has shown the references in their catalog for the comparable DIN, ISO and ASME standards. All stocking metric fastener suppliers are encouraged to consider doing the same if they are not already doing so.

It is also understood that many end users are still referring to DIN fastener standards because they are unaware of DIN withdrawing their standards in preference for the ISO standards. Suppliers are encouraged to suggest that end users adopt ISO fastener standards for all new designs and transition into ISO standards (away from the DIN standards), where possible, for current usage.

There is no law or rule that states users cannot continue to use a withdrawn standard forever if they wish to do so. They should, however, realize that of the DIN, ASME, and ISO metric fastener standards, only the ISO will be technically maintained and updated in the future.

ISO metric fastener standards are the only true international fastener standards.

It is not good engineering practice to have more than one standard for any given product. Multiple metric fastener standards cause unnecessary confusion and adversely affect product uniformity. U.S. industry has been in a state of confusion since the 1970s with its use of metric fasteners, because users have been given three standards to choose from for essentially the same products.It is time for all companies in the supply chain of metric fasteners to start an orderly transition towards the uniform adoption of fasteners made to the ISO fastener standards. To effectively compete in worldwide commerce, U.S. product manufacturers need to utilize the worldwide-accepted ISO fastener standards.

JOE GREENSLADE

Joe Greenslade is director of engineering technology for the Industrial Fasteners Institute (IFI). He has been active in various capacities in the fastener industry since 1970. Before joining the IFI in 2007, he held sales, engineering, and management positions in fastener manufacturing firms and owned his own company for 28 years, providing products and services to fastener producers throughout the U.S. Mr. Greenslade is a speaker, author and consultant on topics related to the fastener industry. He is an active member and leader in standards-related activities at ASME, ASTM, SAE and ISO.